WOR-64: retire integrity-check from WorkBundle

[Ldsystem]

Summary

• hard-retire the unused integrity-check skill, rules, references, report CLI, and helper code
• remove integrity-check from rule-authoring scopes and documentation
• preserve the unrelated merge-skill-hints command in the skill-registry module
• add negative regression coverage for the retired runtime and authoring surfaces

Validation

• RED: 2 retirement tests failed against the old surface
• GREEN: 2 retirement tests passed
• focused contracts: 52 passed
• broader suite excluding a pre-existing registry-layout harness failure: 616 passed, 3 skipped
• toolkit rule validation: passed
• instruction audit: reported successfully
• full suite: 624 passed, 3 skipped, 9 unrelated registry-layout failures; representative failure reproduces on clean main

Linear: https://linear.app/workbundle/issue/WOR-64/retire-integrity-check-from-the-workbundle-toolkit
Discussion: https://workbundleworkspace.slack.com/archives/C0BSA4ASR32/p1788087070915629